The present invention relates to a system for controlling a shift capacity of an automatic transmission, i.e. an engagement capacity of friction elements to be operated during shifting of the automatic transmission.
By selective hydraulic actuation or engagement of the friction elements such as a clutch, brake, etc. which determine a transmission route of a planetary gear, the automatic transmission selects a corresponding speed, and changes power out of an engine in accordance with the gear ratio corresponding thereto. Shifting of the automatic transmission from one speed to another is carried out by changing combination of the friction elements.
For this reason, if the engagement capacity of the friction elements engaged for this shifting is too large, great shift shock is produced by engagement of the friction elements. It is noted that the engagement capacity of the friction elements is determined by a working pressure thereof. Contrarily, if the engagement capacity of the friction elements is too small, slippage of the friction elements is produced, resulting in a decreased longevity of the automatic transmission. It is thus understood that the engagement capacity of the friction elements in particular during shifting, i.e. the shift capacity of the automatic transmission should be controlled appropriately. In view of such circumstances, a technique of controlling a line pressure serving as a source pressure of the whole automatic transmission has been proposed as shown, for example, in JP-A 1-169164.
According to this, a time that the effective gear ratio indicated by the ratio of input revolution to output revolution of the planetary gear changes from a value before shifting to a value after shifting, i.e. an inertia phase time, is measured, and the line pressure is controlled based on self-learning so that the inertia phase time has an optimum value in terms of countermeasures against shift shock.
With such conventional shift capacity control system, when involvement of the friction elements is unpreferable at an initial stage of engagement, which tends to occur when the friction element is a band brake, shorter inertia phase time is measured, which results in judgment that the line pressure is too high, i.e. the shift capacity is too large, carrying out control for decreasing the line pressure. However, since unpreferable involvement of the friction elements means insufficient shift capacity, a continuous decrease in the line pressure or shift capacity based on such erroneous judgment of the shift capacity comes to finally a lower limit value, producing impossible control.
Likewise, when shifting of the automatic transmission is extended since a shelf value of a second speed selecting pressure is low due to low line pressure, shorter inertia phase time is measured, which results in judgment that the line pressure is too high, i.e. the shift capability is too large, carrying out control for decreasing the line pressure. However, since the low shelf value of the second speed selecting pressure means insufficient shift capacity, a continuous decrease in the line pressure or shift capacity based on such erroneous judgment of the shift capacity comes to finally a lower limit value, producing impossible control.
It is, therefore, an object of the present invention to provide a shift capacity control system for an automatic transmission which allows appropriate control of the shift capacity thereof.